The present invention relates to a direction finding antenna system. More particularly, it relates to cable calibration of a four-element array of the interferometer type particularly suited for determining the relative bearing of an intruding aircraft from a protected aircraft in conjunction with the Traffic Alert Collision Avoidance System (TCAS).
Traffic Alert and Collision Avoidance Systems (xe2x80x9cTCASxe2x80x9d) are a well-known means for protecting aircraft in flight from other aircraft, commonly referred to as intruding aircraft, in the vicinity of the protected aircraft. During flight, TCAS equipment located aboard a protected aircraft periodically transmits interrogation signals. Such interrogations may occur at a rate between 38 and 200 per second, depending upon traffic conditions. These interrogations are received by Air Traffic Control Radar Beacon Systems (xe2x80x9cATCRBSxe2x80x9d) or Mode Select (xe2x80x9cMode Sxe2x80x9d) altitude reporting transponders located aboard intruder aircraft. The transponder aboard the intruding aircraft transmits a reply signal which reports its altitude. The TCAS equipment computes the range of the intruding aircraft by using the round-trip time between the transmission of the interrogation and the receipt of the reply.
Altitude, altitude rate, range and range rate are determined by tracking replies to successive interrogation signals. These data are used to determine whether the intruding aircraft is a threat. If threat detection logic in the TCAS computer determines that an intruder aircraft presents a potential collision or near-miss encounter, computer threat resolution logic determines an appropriate vertical maneuver, such as climb, dive, or maintain altitude, that will ensure the safe vertical separation and minimum deviation from the protected aircraft""s current vertical rate.
The TCAS described in document RTCA/DO-185 MINIMUM OPERATIONAL PERFORMANCE STANDARDS FOR TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEM, published September, 1983, by the Radio Technical Commission for Aeronautics, Washington, D.C., provides advisories only for vertical maneuvers of the protected aircraft to escape collision with an intruder aircraft. An improved TCAS has been demonstrated to provide additional advisories for horizontal turning maneuvers, thereby improving collision avoidance. Improved TCAS requires knowledge of the bearing of the intruding aircraft relative to the protected aircraft, in addition to the other data collected by TCAS.
Various antenna systems have been devised for determining relative bearing of an intruder aircraft. For example, U.S. Pat. No. 4,414,550 entitled LOW PROFILE CIRCULAR ARRAY ANTENNA, issued to Tresselt on issued Nov. 8, 1983, discloses an eight element circular array antenna with direction finding capability by virtue of a Butler beam forming matrix used in conjunction therewith. An antenna of this type has been used successfully in the improved TCAS to determine relative bearing of intruding aircraft.
In another example, U.S. Pat. No. 3,792,472, entitled Warning Indicator To Alert Aircraft Pilot Presence And Bearing Of Other Aircraft, issued to Payne et al on Feb. 12, 1974, discloses a five element antenna system for receiving standard ATCRBS reply signals from intruding aircraft and providing approximate relative bearing. Payne et al utilizes an antenna having four elements spaced at 90-degree intervals about the circumference of a circle with a fifth element at its center. The bearing of an intruding aircraft is determined, within an accuracy of +/xe2x88x9222.5-degree, by individually comparing the phases of the signals received by the circumferential elements with that of the signal received by the center element.
The TCAS disclosed in U.S. Pat. No. 4,855,748, entitled TCAS Bearing Estimation Receiver Using A 4 Element Antenna, issued to Brandao et al on Aug. 8, 1989, the complete disclosure of which is incorporated herein by reference, utilizes an interferometer array to determine relative bearing of an intruder aircraft. Briefly, the Brandao et al patent discloses using the phase difference between antenna elements of an incoming transponder radio frequency (xe2x80x9cRFxe2x80x9d) reply to measure bearing angle of the reply, commonly referred to as the relative angle of arrival (xe2x80x9cAOAxe2x80x9d). When an interferometer array is used to determine bearing angle, the phase errors of the measurement means cannot be ignored. Such phase errors cannot be entirely accounted for by factory calibration of the equipment, since the phase errors change with the age, temperature and other variables of the equipment. One error source is the difference in the phase delay through the receiver channels supplying inputs to the phase detector. Above incorporated U.S. Pat. No. 4,855,748 describes a TCAS receiver calibration that overcomes this receiver pair phase delay error.
Differences between the phase delays of transmission lines may also produce significant errors in the outputs of the analog phase detectors. U.S. Pat. No. 4,855,748 also describes a TCAS cable calibration that overcomes the problem of correcting for the difference in phase delay the transmission lines by injecting isophase signals from a test oscillator operating at the nominal transponder transmission frequency of 1090 MHz into each of the dipole antenna elements. However, current aircraft interferometer array installations still require the antenna cables to be phase matched within xc2xd wavelength in order to accurately measure the transponder reply signal AOA. Phase matching the cables minimizes phase measurement errors caused when the transponder reply frequency differs from the TCAS calibration frequency within the allowable variation of +/xe2x88x923 Mhz. Cable phase matching leads to higher installation costs and higher maintenance costs when cables are repaired or replaced. Thus, eliminating the need for antenna cable phase matching in an interferometer array is desirable.
Furthermore, inaccuracies inherent in the analog phase detectors of the system are additional sources of error in the bearing angle measurement. In U.S. Pat. No. 4,855,748, these inherent inaccuracies are accounted for by calibrating the phase detectors at system power-up and periodically thereafter during operation. Samplings of the phase detector outputs during calibration provide data for a look-up table used to correct the bearing measurement output of the phase detectors for the entire angular range of the detectors.
Briefly, the invention comprises a direction finding receiving system on a protected aircraft in which a signal from an ATCRBS transponder on an intruder aircraft is received by a four-element interferometer type antenna array. The antenna elements are disposed in two pairs along perpendicular axes, with the axis of one pair preferably aligned with the heading axis of the aircraft. Each element of the array is connected to an individual receiver, the outputs of which are connected to a broadband digital phase detector.
According to the invention, the broadband digital phase detector is used to calibrate the transmission channels, including the antenna cables and receivers, at the nominal and extremes of the allowable transponder reply frequency variation. When implemented in a TCAS, the nominal transponder reply frequency is 1090 MHz and the extremes of the allowable transponder reply frequency variation are 1087 MHz and 1093 Mhz, respectively. From this calibration data the phase correction at all frequencies from 1087 MHz to 1093 Mhz is calculated.
The invention also uses a frequency discriminator to measure the frequency of each transponder RF reply, and uses this frequency to select the proper phase correction to be used for that reply. The difference in phase between adjacent samples of the digital phase detector is measured and used to determine the frequency. The invention results in a more accurate angle of arrival measurement versus frequency and negates the need for antenna cable phase matching and receiver phase matching.
According to one aspect of the invention, the invention provides a direction finding antenna system capable of determining the relative bearing of a received signal. The system includes a four-element interferometer-type antenna system with each antenna element coupled to a corresponding receiver that amplifies, converts, and limits the signal received thereon and outputs a 48 MHz intermediate frequency (I.F.) output signal. The outputs of the respective receivers are coupled to respective analog-to-digital converters. The analog-to-digital converters output sequential intermediate frequency signal samples of the received signal. The outputs of the individual analog-to-digital converters are simultaneously applied to different variable phase input ports of a digital phase detector. The digital phase detector also includes a reference phase input port that is coupled to a reference phase generator to receive a reference phase output signal thereof. The digital phase detector generates I and Q data by comparing the reference phase against the signals received by respective antenna elements. The I and Q data are output to a processor, such as a microprocessor. The processor includes a frequency discriminator that compares sequential output signals of respective analog-to-digital converters and determines a frequency of each of the output signals. The processor determines phase corrected values of the I and Q data and determines an angle of arrival of the received signal as a function of the phase corrected values.
According to another aspect of the invention, the direction finding antenna system includes a synthesizer that generates various calibration signals at different frequencies in response to a command from the microprocessor. The direction finding antenna system also includes a transmitter coupled to the synthesizer and to one of the antenna elements. The transmitter injects a calibration signal into the antenna element.
According to another aspect of the invention, a switch under the control of the microprocessor is coupled to the transmitter and each of the antenna elements in such manner that the calibration signals are alternately applied to different antenna elements.
According to still another aspect of the invention, the direction finding antenna system includes an input memory coupled to the processor and having correction data stored therein. The processor accesses the correction data to determine the phase corrected values of the I and Q data. The correction data stored in the input memory is preferably accessed by the processor as a function of the frequency of the receiver output signals.
According to various aspects of the invention, the correction data further includes either one or more correction factors, or one or more phase corrected values. Preferably, the calibration signals include at least a first signal having a low frequency relative to a nominal value of the received signal and a second signal having a high frequency relative to that nominal value. The low frequency further is preferably a minimum of an allowable range of received signal frequency values and the high frequency further is preferably a maximum of the allowable range of received signal frequency values. When practiced in a Traffic Alert Collision Avoidance System, the minimum and maximum of an allowable range of received signal frequency values are 1087 MHz and 1093 MHz, respectively.
According to various other aspects of the invention, a method for using a direction finding antenna system for determining the relative bearing of a received signal is provided. The method includes receiving a signal into each of four spaced apart antenna elements mounted in cruciform configuration. The receivers amplifying, converting and limiting the received signal and forming an analog intermediate frequency output signal therefrom. Using analog-to-digital converters to convert the analog intermediate frequency output signal into a digital intermediate frequency output signal and outputting sequential samples of the digital intermediate frequency output signal. In a digital phase detector, comparing the phase of the digital intermediate frequency output signal samples with a reference phase to form in-phase and in-quadrature data therefrom. Determining a frequency of the digital intermediate frequency output signal samples with a frequency detector. In a microprocessor, determining phase corrected values of the in-phase and in-quadrature data, and determining a first quantity related to the relative bearing to the received signal as a function of the phase corrected values. The referenced quantity being an angle-of-arrival of the received signal to the antenna system.
According to various aspects of the method of the invention, determining phase corrected values of the in-phase and in-quadrature data further includes accessing a stored file of correction data, preferably as a function of the frequency of the digital intermediate frequency output signal samples. According to one aspect of the invention, the phase corrected values of said in-phase and in-quadrature data is determined by arithmetically combining predetermined correction data with the in-phase and in-quadrature data formed from the digital intermediate frequency output signal samples and reference phase. Alternatively, according to another aspect of the invention, the phase corrected values of said in-phase and in-quadrature data is determined by substituting correction data that includes alternative predetermined in-phase and in-quadrature data having the phase corrections included therein.
According to still other aspects of the invention, a method is provide for calibrating a direction finding antenna system capable of determining the relative bearing of a received signal. The calibration method of the invention includes injecting a calibration signal into one of the of antenna elements, and receiving the calibration signal into others of the antenna elements. A receiver is used for amplifying, converting and limiting the received calibration signal, and for forming an analog intermediate frequency output signal therefrom. Analog-to-digital converters coupled to each receiver convert the analog intermediate frequency output signal into respective digital intermediate frequency output signals, and under the command of a microprocessor, output sequential samples of the respective digital intermediate frequency output signals. A frequency detector determines the frequency of respective digital intermediate frequency output signal samples. A digital phase detector is used for comparing the phase of the digital intermediate frequency output signal samples with a reference phase to form in-phase and in-quadrature data therefrom. The microprocessor determines phase correction data corresponding to the in-phase and in-quadrature data and stores the same in an input memory coupled thereto.
According to one aspect of the method of the invention for calibrating a direction finding antenna system, the storing of phase correction data further includes storing the phase correction data as a function of the corresponding intermediate frequency.
According to one aspect of the calibration method of the invention, the injecting of a calibration signal includes injecting multiple calibration signals. Preferably, the calibration signals include at least a minimum and a maximum of an allowable range of frequencies of the received signal.
According to another aspect of the calibration method of the invention, the of a calibration signal also includes alternately injecting the calibration signal into two or more different antenna elements.